ok i have an old 1983 fb 12a. im about to buy a GSL_SE. so is it possible to switch the whole gauge from the 12a to the 13b so the rpm is in the middle? im hoping they both have the same wires and i could just plug them right in.

 

If you mean switching the guage pod, no, it won't fit. The unit on the 79-83 is more rounded, where the one on the 84-85 is angular.